The present invention is directed to reflectors used in the radiant section of a fired heater, and more particularly to ceramic reflectors provided on a refractory wall centered in the spacing between the radiant tubes.
Combustion equipment is generally operated in chemical plants, petrochemical plants and refineries. The equipment may include industrial heaters, furnaces or plant boilers. This equipment is generally designed with bare or smooth-walled tubes, or with partially studded tubes as disclosed in my U.S. Pat. No. 6,364,658, which is hereby incorporated by reference in its entirety. Use of tubes in radiant sections usually exposes the front half of the tube to direct flame radiation, while limiting the exposure of the rear half or dark side of the tube to reflected radiation.
The heat flux distribution around the circumference of a conventionally fired tube at a conventional spacing of two tube diameters is depicted in FIG. 1. A flame or radiating plane is on one side of the tube and a refractory wall is on the other. The front half of the tube surface faces the flame (point A) and receives a higher heat flux as compared to the rear half facing the refractory wall (point B). Point A receives heat flux only from direct flame radiation, while point B, facing the refractory wall, receives only reflected radiation coming from the refractory wall. Points between point A and point B receive varying amounts of both direct and reflected radiation, depending upon their location along the tube.
The standard distance between tubes is two tube diameters from center-to-center, and 1.5 diameters from the center of the tubes to the refractory wall, for most operations in the chemical and petrochemical industries, as shown in FIG. 2. The heat flux distribution in FIG. 1 is based on this configuration. For the purposes of an illustration using fluxes typical in a conventional fired heater, where the highest heat flux at point A is 18000 Btu/hr-ft2, the diametrically opposed counterpart (point B) receives only 6000 Btu/hr-ft2. The rear half of the tube transfers only 24% of the total heat absorbed by the tube; this includes both the direct and reflected radiation, as seen in FIG. 3. The average flux for the tube amounts to 10,000 Btu/hr-ft2.
More than 85% of the heaters in the industry have such a large flux differential between the front and the rear side of the tube, as this illustration depicts. A significant compromise is made on the overall heat-receiving capacity of the tube in order to keep the flame-front side (point A) within safe working temperatures.
To make the heat flux distribution in the tube more uniform, one approach of the furnace designers has been to increase the center-to-center tube spacing requirements from 2 to 3 tube diameters. This design increases the flux at point B of the tube from 6,000 Btu/hr-ft2 to 9,000 Btu/hr-ft2 as shown in FIGS. 4A and 4B. The increased spacing has the beneficial result of increasing the heat-receiving capacity of the rear half of the tube for the 3D-spaced tubes, while heat flux distribution on the front half of the tube is generally the same as for the 2D-spaced tubes. This results in an increase of the average heat flux to 12,000 Btu/hr-ft2 for the entire tube. However, the drawback of this solution is apparent. With an increase in tube spacing there is a corresponding increase in the size of the heater. This increases the cost and space requirements for the heater.
Another prior art approach improves the heat flux distribution by placing radiating flames on opposing sides of the tubes in a so-called “double-fired” design. A comparison is shown between one radiating flame (A) and two radiating flames (B) in FIGS. 5A and 5B, respectively. This design is commonly used in chemical processes that mandate a more uniform heat flux distribution, such as, for example, in delayed cokers, high-pressure hydrotreaters, ethylene furnaces, and the like. In a double-fired system, the front (point A) and rear (point B) portions of the tube have the same heat flux rate due to direct flame radiation, and the points at the margins between the front and rear receive relatively less direct flame radiation. The corresponding distribution of the heat flux, for the illustrative example, is 18,000 Btu/hr-ft2 for the front and the rear locations, 13,500 Btu/hr-ft2 at the margins between the front and rear faces, i.e. the middle area of the tube (point M at the 90 and 270 degree positions), resulting in an average flux of 15,000 Btu/hr-ft2. The double-fired design brings with it the disadvantage that the heater has to be much larger, as much as twice the size as a single-fired unit, and correspondingly more expensive.
The present state of technology for heaters with a standard spacing of 2 tube-diameters will have a relative flux ratio of 1 to 1.8 between the average flux and the maximum flux, whereas a heater with a 3 tube-diameter spacing will have a relative flux ratio of 1 to 1.5, as shown in API Standard 530, Calculation of Heater-Tube Thickness in Petroleum Refineries, American Petroleum Institute (1988), Figure C-1 Ratio of Maximum Local to Average Heat Flux Curves, page 103.
The 3 tube-diameter design is less common in the industry and the vessel must be significantly larger than a 2 tube-diameter design. The average to maximum flux ratio of the double-fired tubes is significantly lower at 1 to 1.2, but is a more costly alternative of the three designs for an industrial plant.
A recent improvement in the flux distribution as described in my “458 patent involves the placement of extended surfaces such as studs or fins on the dark side of the tubes in a single-fired arrangement. This improves the heat transfer to the dark side of the tubes primarily by increasing the convection heat transfer. Still, in the standard tube arrangement with smooth walls, it is well known that 65.8% of the radiant heat from the flame is absorbed by the tubes, primarily the front half of the tubes facing the flame, and 34.2% goes through the spaces between the tubes to the refractory wall. The same percentages apply to the reflected radiation from the refractory onto the dark side of the tubes, i.e. 65.8% of the 34.2% is re-radiated to the rear half of the tubes, or 22.5%. In other words, 88.3% is absorbed by the tubes, front and back, and the balance of 11.7% is radiated back to the flame through the spaces between the tubes. It would be very desirable if a significant portion of this 11.7% could be directed onto the tubes instead of the flame. There thus remains a need for making the flux distribution even more uniform and/or for increasing the rate of heat absorption by the tubes.